Victory points are added up at the end of the game as a way of determining who has won the battle. To work out your victory points total you must add together the points value of the enemy units you have destroyed or seriously damaged.
Each unit wiped out is worth its points cost as victory points to the enemy. A unit which falls back off the edge of the table or is falling back at the end of the game counts as being wiped out.
If a unit has lost half or more of its models then it is worth half its total points value as victory points. If it has lost less than half of its models then it is worth no victory points to the enemy.
Vehicles which have been damaged (immobilised or armament destroyed) are worth half their points value, those which have been destroyed are worth their full points value. Even if you are using a unit of several vehicles (like Eldar Vypers, for example) count victory points for each vehicle separately. For example, if a unit of two Eldar Vypers suffered one damaged and one destroyed the enemy would get full victory points for one and half victory points for the other.
Individual models with two or more Wounds are worth half their points value if reduced to half Wounds, and their full value if killed.
As well as the points for destroying the enemy, many missions award additional victory points for successfully completing objectives during the game. These extra victory points are noted in the relevant mission's description and should be included in each player's total at the end of the game.
Add together the points values of everything destroyed, damaged or below half strength for each side. Add to that any points for achieving mission objectives. The side with the most victory points wins the game.